1. Field of the Invention
The present invention relates to an optical pickup apparatus in an optical recording medium player, and, more particularly, to an optical pickup apparatus which includes a focus error detector and a tracking error detector.
2. Description of the Related Art
In general, disk players which play an optical recording medium, such as an optical video disk or a digital audio disk, (hereinafter simply called "disk") inevitably require so-called focus servo and tracking servo to always converge an information reading light beam correctly on the recording surface of a disk. The focus servo controls the position of the objective lens in the direction of the optical axis of the objective lens, which irradiates a light beam on the recording surface of the disk, so as to reduce the positional error of the objective lens to;the focus position along the optical axis of the objective lens or reduce a focus error. The tracking servo controls the position of the objective lens, which irradiates a light beam on the recording surface of the disk, in the radial direction of the disk with respect to the recording tracks so as to reduce the positional error of the objective lens to the recording tracks or reduce a tracking error.
FIG. 1 illustrates the structure of an optical pickup apparatus including those servo systems.
In the diagram, the laser beam from a semiconductor laser 1, which generates a single read beam, is converted into a parallel laser beam by a collimator lens 2. This parallel laser beam is irradiated via a beam splitter 3 to an objective lens 4. The objective lens 4 condenses this parallel laser beam into a focused laser beam and irradiates this beam toward an optical disk 5. This focused laser beam is reflected by the optical disk 5, and the thus reflected laser beam is converted into a parallel laser beam by the objective lens 4. The parallel laser beam enters the beam splitter 3 to be directed to a polarizing beam splitter (hereinafter, referred to as PBS) 6. The PBS 6 separates the light beam from the beam splitter 3 into P and S polarized lights. The P polarized light from the PBS 6 is condensed to be a focused laser beam by a detection lens 7. This focused laser beam passes through a cylindrical lens 8, forming a spot on four light-receiving surfaces of a quarter-split photosensor 9. The quarter-split photosensor 9 has four light-receiving surfaces defined by a pair of lines crossing perpendicularly to each other. Based on this focused laser beam, the cylindrical lens 8 generates an astigmatic beam. When the focused laser beam irradiated on the optical disk 5 by the objective lens 4 is in focus, the cylindrical lens 8 irradiates spot light SP of a true circle, as shown in FIG. 2(a), on the quarter-split photosensor 9. When the focused laser beam is out of focus, the cylindrical lens 8 irradiates ellipsoidal spot light SP as shown in FIG. 2(b) or 2(c) on the quarter-split photosensor 9. This spot light SP has an ellipsoidal shape in the diagonal direction of the elements of the photosensor 9. The S polarized light reflected by the PBS 6 passes through a detection lens 10, forming a spot on two light-receiving surfaces of a half-split photosensor 11. The two light-receiving surface of the photosensor 11 are defined by the line that bisecting the entire light-receiving surface of the photosensor 11. The quarter-split photosensor 9 photoelectrically converts individual portions of the spot light on the four light-receiving surfaces into electric signals and supplies the electric signals to a focus error detector 12. Based on those received electric signals, the focus error detector 12 produces a focus error signal (FE) and sends the error signal to an actuator driver 13. The half-split photosensor 11 photoelectrically converts individual portions of the spot light on the two light-receiving surfaces into electric signals and supplies the electric signals to a tracking error detector 14. Based on those received electric signals, the tracking error detector 14 produces a tracking error signal (TE) and sends the error signal to the actuator driver 13. The actuator driver 13 produces a focusing drive signal to move the objective lens 4 in accordance with the focus error signal, and produces a tracking drive signal to also move the objective lens 4 in accordance with the tracking error signal. The actuator driver 13 sends those drive signals to an actuator 15. The actuator 15 moves the objective lens 4 along the optical axis in accordance with the focusing drive signal, and moves the objective lens 4 in the radial direction of the recording tracks in accordance with the tracking drive signal.
FIG. 3 illustrates the structure of an error signal generator 20, which comprises the aforementioned quarter-split photosensor 9, half-split photosensor 11, focus error detector 12 and tracking error detector 14.
The tracking error detector 14 will be described first.
The half-split photosensor 11 comprises two independent elements 11a and 11b whose photoelectrically converted outputs are supplied to a differential amplifier 14a that constitutes the tracking error detector 14. The differential amplifier 14a produces a difference between the photoelectric conversion outputs from the elements 11aand 11b as the tracking error signal (TE). The bisecting line for the two elements of the half-split photosensor 11 optically matches with the track direction of the disk. When the laser beam is on a track, the amounts of light at the two light-receiving surfaces are equal to each other. When the laser beam is off a track, there is a difference between those light amounts at the two light-receiving surfaces.
Now the focus error detector 12 will be described.
The quarter-split photosensor 9 comprises four independent elements 9a to 9d, which are located adjoining to one another with two split lines L.sub.1 and L.sub.2 as boundaries. One of the lines, L.sub.1, is parallel to the track direction. The photoelectric conversion outputs of the elements 9a and 9c, symmetrical to each other with respect to the center of the light-receiving surfaces 0 of the quarter-split photosensor 9, are added by an adder 21. Likewise, the photoelectric conversion outputs of the elements 9b and 9d, also symmetrical to each other with respect to that center O, are added by an adder 22. The added outputs of those adders 21 and 22 are supplied to a differential amplifier 23 respectively via variable resistors VRa and VRb. The differential amplifier 23 computes the difference between the signals supplied via the variable resistors VRa and VRb and sends a signal representing that difference to a low-pass filter (hereinafter, referred to as LPF) 24. The LPF 24 extracts a signal with a lower frequency than a predetermined cutoff point fc among the signal supplied from the differential amplifier 23, and sends the low-frequency signal to an adder 25. Further, the photoelectric conversion outputs of the elements 9a and 9c are supplied to an adder 26 respectively via variable resistors VRc and VRd, and the photoelectric conversion outputs of the elements 9b and 9d are supplied to an adder 27 respectively via variable resistors VRe and VRf. The adder 26 adds the signals received via the variable resistors VRc and VRd and sends a resultant sum signal to a differential amplifier 28. The adder 27 adds the signals received via the variable resistors VRe and VRf and sends a resultant sum signal to the differential amplifier 28. The differential amplifier 28 computes the difference between the signals from the adders 26 and 27, and sends a signal representing that difference to a high-pass filter (hereinafter, referred to as HPF) 29. The HPF 29 extracts a signal with a higher frequency than the predetermined cutoff point fc among the signal supplied from the differential amplifier 28, and sends the high-frequency signal to the adder 25.
The adder 25 adds the signals from the LPF 24 and HPF 29, and produces a resultant sum signal as the focus error signal (FE). The characteristics of the LPF 24 and the HPF 29 are complementary to each other, and the cutoff point fc is set sufficiently low with respect to the servo band such that the point may not affect the servo loops; for example, fc is set to about 10 Hz. 0n the low-frequency side, the focus balance is adjusted by the variable resistors VRa and VRb. On the high-frequency side, the level control is effected by the variable resistors VRc to VRf so that the levels of the outputs of the four elements become the same.
In the focus error detector 12, as described above, the photoelectric conversion outputs of every diagonally adjoining two of the four elements are added by the adders 21 and 22 (adders 26 and 27) respectively, and the difference between the results of those two additions is acquired by the differential amplifier 23 (differential amplifier 28), thus generating a focus error component. When the focused laser beam is in focus, the spot light of a true circle as shown in FIG. 2(a) is formed on the quarter-split photosensor 9. Therefore, the result of the addition of the photoelectric conversion outputs of one diagonal pair of elements among the four elements equals the result of the addition of the photoelectric conversion outputs of the other diagonal pair of elements, and the focus error component becomes "0". When the focused laser beam is out of focus, the ellipsoidal spot light as shown in FIG. 2(b) or 2(c) is formed on the quarter-split photosensor 9. Therefore, the result of the addition of the photoelectric conversion outputs of one diagonal pair of elements differs from the result of the addition of the photoelectric conversion outputs of the other diagonal pair of elements. Thus, the focus error component output from the differential amplifier 23 (differential amplifier 28) has a value corresponding to that focus deviation.
The thus produced focus error component is temporarily separated into a low-frequency component and a high-frequency component by the LPF 24 and HPF 29. At this time, as the focus error component has a relatively long fluctuation period, it is extracted as a low-frequency component by the LPF 24. There may occur a crosstalk component or noise which is produced by the signal superimposition when a beam spot traverses the tracks in a special mode, such as a scan or still mode. As the crosstalk component has a relative short fluctuation period, it is extracted as a high-frequency component by the HPF 29. Those crosstalk component and focus error signal component are added by the adder 25, yielding a final focus error signal (FE). Therefore, the above-described focus error detector can generate a focus error signal (FE) which is free of the crosstalk component or high-frequency noise which is superimposed on the recorded signal when a beam spot traverses the tracks in a special mode, such as a scan or still mode.
In the above-described conventional optical pickup apparatus, however, the crosstalk component may not be properly separated and extracted in some cases due to the phase shift of the filters themselves such as LPF 24 and HPF 29, which are used to extract the crosstalk component. Accordingly, the conventional apparatus cannot always produce a focus error signal free of such a crosstalk component.